The detection and measurement of electric currents through application of the general principle of induction is well known, as exemplified by clamp-on meters used for relatively low frequency current measurement. Briefly, these known devices incorporate a magnetic material loop arranged to encircle a current carrying conductor, which loop has an electric potential induced therein that can be detected by a galvanometer, the latter being optionally calibrated for direct readout. At low frequencies, these devices have found wide use and are relatively insensitive to secondary inductions and do not require precise location of the current carrying conductor with respect to any of the detection equipment parts.
However, as the frequency of the current being measured increases, various factors which can be substantially ignored at lower frequencies must now be taken into account in order to obtain accurate determinations. First of all, although conductor resistance loss is responsible for most of the attenuation at low frequencies, at higher frequencies dielectric loss is the primary cause of attenuation. Moreover, the series resistance of a radio frequency (or higher) line is controlled by a physical phenomenon referred to as "skin effect" and it can be shown to be proportional to the square root of the frequency. Still further, the coaxial characteristic impedance is substantially independent of frequency ranging between 20-300 ihms for coaxial conductors or transmission lines. The combined effect of all of these factors makes current detection and measurement more difficult as the frequency increases.
In addition to desiring to unobtrusively determine the magnitude of coaxial transmission line current, there are situations in which it is advantageous to be able to test the effectiveness of such apparatus as cable termination means. Exemplary of what is referred to here, there are many environments (e.g., aboard a ship) where shielded cables are exposed to relatively large interference electromagnetic fields which induce correspondingly large interference currents in the cable shield. If these currents are not terminated satisfactorily, they can impair or even destroy the equipment to which the cables are connected. Termination means of considerable variety have been devised to achieve termination for cables as well as other equipment and devices, and it is a desideratum to have non-invasive test equipment which can determine the effectiveness of a particular termination or grounding device prior to its installation and actual on site utilization. An essential part of apparatus for measuring termination means effectiveness is a wide range electrical current detection and measurement device as described herein.